As anyone who had the luck to see Saturday, Sunday, Monday or Napoli Milionaria will know, Eduardo de Filippo was one of the great figures of European theatre. But his penultimate play, written in 1960 and here getting its British premiere in a new translation by Mike Poulton, strikes me as one of his lesser pieces. Full of pious good intentions but awkward stagecraft, it is chiefly memorable for a glorious central performance from Ian McKellen.

McKellen plays Don Antonio: a Neapolitan godfather who rules the city's underworld with a whim of iron but who sees it as his mission to dispense rough justice to victims of civic corruption. So we see Don Antonio rescuing the poor from exploitative loan-sharks, intervening in territorial gun battles and arbitrating in a bitter dispute between a baker and his son.

Although the doctor-friend who has stood by Don Antonio for 35 years sees him as barking mad, we are clearly meant to admire this quixotic dreamer who envisages a world "with no dark corners". But what undermines the play is less its sentimentality than the technical clumsiness of the final act chiefly there, you feel, to provide Eduardo with a good death scene and enable him to get his message across.

Still, there is always the acting, and McKellen gives superb value as this unquiet Don. First seen shadow-boxing in his dressing gown, he captures perfectly the sleek vanity of this septuagenarian overlord. McKellen also has the instinctive authority of a man used to being obeyed: he prowls round his living room like a tiger, greets his supplicants with lordly concern, and has the vocal huskiness that seems inseparable from Mafia bosses.

True to Eduardo's intention, McKellen also suggests that this particular capo, tormented by a killing he committed in his youth, is a thwarted idealist: a Robin Hood in Armani clothing who wants to clean up the streets of Naples and put an end to senseless vendettas. I don't buy the idea but McKellen, with his fluidity of movement and blend of dapperness and danger, executes it perfectly.

Sean Mathias's handsome production also contains a number of other very good performances. Michael Pennington invests the Don's medical sidekick with exactly the right air of terrified loyalty, Oliver Cotton exudes white-suited arrogance as a dictatorial master baker, and Gavin Fowler lends his maltreated son a simmering, murderous resentment. But, although the play is clearly the work of a good man who understood Naples and its people and who devoutly wished for a better world, it's an evening where you rejoice more in the acting than in the over-optimistic message.